Annealing separating agent composition for grain-oriented electrical steel sheet, grain-oriented electrical steel sheet, and method for manufacturing grain oriented electrical steel sheet

ABSTRACT

Provided is an annealing separating agent composition for a grain-oriented electrical steel sheet, a grain-oriented electrical steel sheet and a method for manufacturing a grain-oriented electrical steel sheet. The annealing separating agent composition for a grain-oriented electrical steel sheet according to an embodiment of the present invention contains 30 to 70% by weight of a calcium compound, and the remainder of magnesium oxide or magnesium hydroxide on a solid basis.

TECHNICAL FIELD OF THE INVENTION

The invention relates to an annealing separating agent composition for agrain-oriented electrical steel sheet, a grain-oriented electrical steelsheet, and a method for manufacturing a grain-oriented electrical steelsheet.

BACKGROUND OF THE INVENTION

In general, a grain-oriented electrical steel sheet is a steel sheetcontaining Si component of about 3.1%. Since it has an aggregatestructure in which the grain orientations are aligned in the {100}<001>orientation, it has extremely excellent magnetic properties in therolling direction. Generally known grain-oriented electrical steelsheets are formed by forming an insulating coating on a Forsterite(Mg₂SiO₄)-based coating and applying a tensile stress to the steel sheetusing the difference in thermal expansion coefficient of the insulatingcoating, thereby improving iron loss and reducing noise due tomagnetostriction. However, there is a limit to satisfy thecharacteristic level in the advanced grain-oriented electrical steelsheet which is recently required.

In the conventional grain-oriented electrical steel sheet manufacturingprocess, there has been proposed a method of improving the surfaceproperties of TiO₂ powder by MgO in the step of applying a fusionpreventive agent containing MgO as a main component in order to improvethe properties of the Forsterite coating. Further, a method of removingthe Forsterite coating by applying a mixture of alumina powder orcolloidal silica and MgCl₂ as a fusion preventive agent is known as amethod for improving the iron loss of a grain-oriented electrical steelsheet. However, such a method has a problem that, due to the removal ofthe Forsterite coating, the iron loss of the electrical steel sheet israther disadvantageous. Further, there is a problem that it is difficultto form an insulating coating in a subsequent process.

DETAILS OF THE INVENTION Problems to be Solved

Provided is an annealing separating agent composition for agrain-oriented electrical steel sheet, a grain-oriented electrical steelsheet, and a method for manufacturing a grain-oriented electrical steelsheet.

Means to Solve the Problems

An annealing separating agent composition for a grain-orientedelectrical steel sheet according one embodiment of the present inventionmay comprise, based on the solids content, 5 to 70% by weight of acalcium compound, and magnesium oxide or magnesium hydroxide in abalance.

The composition may further include 1 to 10% by weight of the ceramicpowder. The ceramic powder may be at least one selected from the groupconsisting of Al₂O₃, SiO₂, TiO₂, and ZrO₂.

The composition may further include Sb₂(SO₄)₃, SrSO₄, BaSO₄, or acombination thereof in an amount of 1 to 10% by weight.

The calcium compound may be at least one selected from calcium oxide(CaO), calcium hydroxide (Ca(OH)₂), calcium cobalt oxide (Ca₃Co₄O₉),calcium silicate (CaSiO₃), calcium titanate (CaTiO₃), calcium zirconate(CaZrO₃), hydroxyapatite (Ca₅(OH)(PO₄)₃), calcium carbonate (CaCO₃),calcium hydride (CaH₂), calcium carbide (CaC₂), calcium phosphate(Ca₃(PO₄)₂), calcium sulfate (CaSO₄), calcium oxylate (CaC₂O₄), calciumperoxide (CaO₂), and calcium chromate (CaCrO₄) A grain-orientedelectrical steel sheet according to one embodiment of the presentinvention may have a Monticellite coating formed on one side or bothsides of a grain-oriented electrical steel sheet substrate.

The Monticellite coating may include 0.5 to 90% by weight of Ca.

The Monticellite coating may further include Mg in an amount of 3 to 80%by weight, Si in an amount of 3 to 80% by weight, O in an amount of 3 to80% by weight, and Fe in a balance.

The Monticellite coating may have a thickness of 0.1 to 10 μm.

A ceramic layer may be further formed on the Monticellite coating.

The ceramic layer may include a ceramic powder.

The ceramic powder may be at least one selected from Al₂O₃, SiO₂, TiO₂,ZrO₂, Al₂O₃, TiO₂, Y₂O₃, 9Al₂O₃.2B₂O₃, BN, CrN, BaTiO₃, SiC, and TiC.

The ceramic layer may further include a metal phosphate.

The metal phosphate may include at least one selected from Mg, Ca, Ba,Sr, Zn, Al, and Mn.

The grain-oriented electrical steel sheet substrate may include silicon(Si): 2.8 to 6.8 wt. %, aluminum (Al): 0.020 to 0.040 wt. %, manganese(Mn): 0.01 to 0.20 wt. %, antimony (Sb), tin (Sn), or a combinationthereof in an amount of 0.01 to 0.15 wt. %, and Fe and other unavoidableimpurities in a balance.

A manufacturing method of a grain-oriented electrical steel sheetaccording to one embodiment of the present invention may includepreparing a steel slab; heating the steel slab; hot-rolling the heatedsteel slab to produce a hot-rolled steel sheet; cold-rolling thehot-rolled sheet to produce a cold-rolled sheet; decarburizing annealingand nitriding annealing the cold-rolled sheet; applying an annealingseparating agent on the surface of the decarburizing annealed andnitriding annealed steel sheet; and high-temperature annealing the steelsheet applied with the annealing separating agent.

The annealing separating agent may include, based on a solid content, 30to 70% by weight of a calcium compound, and magnesium oxide or magnesiumhydroxide in a balance.

The method may further include a step of forming a ceramic layer on theMonticellite coating after the high-temperature annealing.

The step of forming the ceramic layer may be a step of forming a ceramiclayer by spraying a ceramic powder onto the Monticellite coating.

The step of forming the ceramic layer may be a step of forming a ceramiclayer by applying a composition for forming a ceramic layer including aceramic powder and a metal phosphate on the Monticellite coating.

The step of decarburization annealing and nitriding annealing thecold-rolled sheet may be a step of decarburization annealing andnitriding annealing the cold-rolled sheet simultaneously, or nitridingannealing the cold-rolled sheet after decarburization annealing.

Effects of the Invention

According to one embodiment of the present invention, it is possible toprovide a grain-oriented electrical steel sheet having excellent ironloss and magnetic flux density and excellent adhesion and insulation ofa coating, and a method for manufacturing the same.

BRIEF DESCRIPTIONS OF DRAWINGS

FIG. 1 schematically shows a Monticellite atomic unit structure.

FIG. 2 is a schematic side cross-sectional view of a grain-orientedelectrical steel sheet according to an embodiment of the presentinvention.

FIG. 3 is a flowchart of a method of manufacturing a grain-orientedelectrical steel sheet according to an embodiment of the presentinvention.

FIG. 4 is an X-ray diffraction (XRD) result of the Monticellite coatingprepared in Example 1.

FIG. 5 is a Scanning Electron Microscope (SEM) photograph of thegrain-oriented electrical steel sheet produced in Example 1.

FIG. 6 is a Scanning Electron Microscope based Energy DispersiveSpectroscopy (SEM EDS) analysis result of the Monticellite coatingprepared in Example 1.

FIG. 7 shows Fourier Transform Infrared Spectroscopy (FT-IR) analysisresults of the Monticellite coating and the Fosterite coating preparedin Example 1 and Comparative Example 2.

DETAILED DESCRIPTIONS OF THE INVENTION

The terms first, second, third, and the like are used to describevarious portions, components, regions, layers and/or sections, but arenot limited thereto. These terms are only used to distinguish oneportion, component, region, layer or section from another portion,component, region, layer or section. Thus, a first portion, component,region, layer or section described below may be referred to as a secondportion, component, region, layer or section without departing from thescope of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to limit the invention

The singular forms as used herein include plural forms as long as thephrases do not specifically state the opposite meaning thereof. The“comprises” means that a particular characteristic, region, integer,step, motion, element and/or component is specified and that does notexclude the presence or addition of other characteristics, regions,integers, steps, motions, elements, and/or components.

When referring to a part as being “on” or “above” another part, it maybe positioned directly on or above another part, or another part may beinterposed therebetween. In contrast, when referring to a part being“directly above” another part, no other part is interposed therebetween.

Further, in the present invention, 1 ppm means 0.0001%.

Unless defined otherwise, all terms including technical and scientificterms used herein have the same meaning as commonly understood by one ofordinary skill in the art to which the present invention belongs. Termsdefined in the commonly used dictionary are further interpreted ashaving a meaning consistent with the relevant technical literature andthe present disclosure, and are not to be construed as ideal or veryformal meanings unless defined otherwise.

Hereinafter, embodiments of the present invention will be described indetail so that a person of ordinary skill in the art could easily carryout the present invention. The present invention may, however, beembodied in various forms and should not be construed as limited to theembodiments set forth herein.

The annealing separating agent composition for a grain-orientedelectrical steel sheet according to an embodiment of the presentinvention contains 30 to 70% by weight of a calcium compound, andmagnesium oxide or magnesium hydroxide in a balance on a solid basis.Here, the solid basis means that the solid content excluding thecomponents such as solvent is set to 100% by weight.

The annealing separating agent composition according to an embodiment ofthe present invention is applied to the grain-oriented electrical steelsheet substrate 10 to form the Monticellite coating 20.

Monticellite is an olivine group consisting of the atomic unit structureas shown in FIG. 1. The magnesium ion is present at the M1 site and thecalcium ion is present at the M2 site. In FIG. 1, oxygen (O) isrepresented by a red circle, silicon (Si) by a pink circle, and Ca andMg by a blue circle.

The Monticellite coating 20 has a chemical structure change due to thesubstitution of Ca ion at the M2 site compared to the conventionalForsterite coating. The melting point is lowered, so that the glasscoating formation temperature is lowered in the high-temperatureannealing process. In addition, the Monticellite coating formed in thelow-temperature region has an effect of inhibiting the decomposition ofthe AlN-based inhibitor, which has a decisive influence on the formationof the secondary recrystallization, and thus it can secure excellentmagnetic quality. Also, Monticellite has Mohs hardness lower than thatof Forsterite, and thus has an advantage of excellent coating adhesion.

The calcium compound serves to supply Ca of Monticellite. Unlike theconventional annealing separating agent composition, in an embodiment ofthe present invention, the Monticellite coating 20 is formed on thesteel sheet substrate 10 by adding a calcium compound.

The calcium compound may not be limited as long as it is a compoundcapable of supplying Ca. Specifically, it can be at least one selectedfrom calcium oxide (CaO), calcium hydroxide (Ca(OH)₂), calcium cobaltoxide (Ca₃Co₄O₉), calcium silicate (CaSiO₃), calcium titanate (CaTiO₃),calcium zirconate (CaZrO₃), hydroxyapatite (Ca₅(OH)(PO₄)₃), calciumcarbonate (CaCO₃), calcium hydride (CaH₂), calcium carbide (CaC₂),calcium phosphate (Ca₃(PO₄)₂), calcium sulfate (CaSO₄), calcium oxylate(CaC₂O₄), calcium peroxide (CaO₂), and calcium chromate (CaCrO₄).

The calcium compound may be contained in the annealing separating agentcomposition in an amount of 30 to 70% by weight. When the calciumcompound is contained too small, the Ca content in the Monticellitecoating 20 may be decreased and the iron loss may be deteriorated. Ifthe calcium compound is contained too much, the Ca content in theMonticellite coating 20 may be increased and the corrosion resistancemay be deteriorated. Therefore, calcium compounds may be included in theabove-mentioned range. More specifically, the calcium compound mayinclude 40 to 60% by weight. More specifically, the calcium compound mayinclude 45 to 55% by weight.

The magnesium oxide or magnesium hydroxide serves to supply Mg ofMonticellite. The magnesium oxide or magnesium hydroxide may bemagnesium oxide (MgO). Since the magnesium oxide (MgO) is generallyknown, a detailed description thereof will be omitted.

The annealing separating agent composition for a grain-orientedelectrical steel sheet may further comprise 1 to 10% by weight of aceramic powder. The ceramic powder may be at least one selected fromAl₂O₃, SiO₂, TiO₂, and ZrO₂. When the ceramic powder further contains anappropriate amount, the insulating property of the Monticellite coating20 can be further improved.

The annealing separating agent composition for a grain-orientedelectrical steel sheet may further include 1 to 10% by weight ofSb₂(SO₄)₃, Sr₅O₄, BaSO₄ or a combination thereof. By further includingproper amount of Sb₂(SO₄)₃, SrSO₄, BaSO₄, or a combination thereof, agrain-oriented electrical steel sheet having an excellent surface glossand a very high roughness can be produced.

The annealing separating agent composition may further include a solventfor even dispersion and easy application of the solids. Water, alcohol,and the like can be used as a solvent. 300 to 1000 parts by weight canbe included relative to 100 parts by weight of the solid content. Assuch, the annealing separating agent composition may be in the form of aslurry.

The grain-oriented electrical steel sheet 100 according to an embodimentof the present invention has a Monticellite coating 20 formed on oneside or both sides of the grain-oriented electrical steel sheetsubstrate 10. FIG. 2 is a schematic side cross-sectional view of agrain-oriented electrical steel sheet according to an embodiment of thepresent invention. FIG. 2 shows a case where the Monticellite coating 20is formed on the upper surface of the grain-oriented electrical steelsheet substrate 10.

The Monticellite coating 20 has a chemical structure change due to thesubstitution of Ca ion at the M2 site compared to the conventionalForsterite coating. The melting point is lowered, so that the glasscoating formation temperature is lowered in the high temperatureannealing process to secure the good quality of surface property. Inaddition, the Monticellite coating formed in the low temperature regionhas an effect of inhibiting the decomposition of the AlN-basedinhibitor, which has a decisive influence on the formation of thesecondary recrystallization, and thus it can secure excellent magneticquality. Also, Monticellite has Mohs hardness lower than that ofForsterite, and thus has an advantage of excellent coating adhesion.

The Monticellite coating may contain 0.5 to 90% by weight of Ca. If theCa content in the Monticellite coating 20 is too small, the iron loss ofthe grain-oriented electrical steel sheet may be deteriorated. If the Cacontent in the Monticellite coating 20 is too high, the corrosionresistance may be deteriorated. Therefore, Ca may be included in theabove-mentioned range. More specifically, Ca may be contained in anamount of 4 to 65% by weight.

The Monticellite coating may contain 3 to 80% by weight of Mg. If the Mgcontent is too low, the formation amount of the Monticellite coating isinsufficient, causing surface defects. If the Mg content is too high,Forsterite may be formed and the iron loss property may be deteriorated.Therefore, Mg may be included in the above-mentioned range.Specifically, Mg may be contained in an amount of 5 to 50% by weight.More specifically, Mg may be contained in an amount of 7 to 15% byweight.

The Monticellite coating may contain 3 to 80% by weight of Si. If the Sicontent is too small, the formation amount of the Monticellite coatingmay be insufficient and the adhesion property mat be deteriorated. Ifthe Si content is too high, surface defects of the whitening phenomenonmay occur. Therefore, Si may be included in the above-mentioned range.Specifically, it may contain 5 to 50% by weight of Si. Morespecifically, it may contain 7 to 15% by weight of Si.

The Monticellite coating may contain 3 to 80% by weight of oxygen (O).More specifically, it may contain 5 to 50% by weight of O. Morespecifically, it may contain 7 to 15% by weight of O.

The Monticellite coating may contain Fe as the remainder. Carbon (C) maybe included as an impurity.

The Monticellite is formed by reacting calcium (Ca) and magnesium (Mg),which are the main components of the composition, with silicon (Si)contained in the grain-oriented electrical steel sheet during theapplication of the annealing separating agent composition. TheMonticellite coating 20 has an excellent coating tensioning impartingeffect.

The Monticellite coating 20 may have a thickness of 0.1 to 10 μm. If thethickness of the Monticellite coating 20 is too thin, the coatingtension imparting ability may be lowered to cause a problem fordeteriorating the iron loss. If the thickness of the Monticellitecoating 20 is too thick, the adhesion of the Monticellite coating 20 maybe weakened and may cause peeling. Therefore, the thickness of theMonticellite coating 20 can be adjusted to the above-mentioned range.More specifically, the thickness of the Monticellite coating 20 may be0.8 to 6 μm.

The grain-oriented electrical steel sheet 100 according to an embodimentof the present invention may further include a ceramic layer 30 on theMonticellite coating 20. FIG. 2 shows an example in which a ceramiclayer 30 is further formed on the Monticellite coating 20.

The thickness of the ceramic layer 30 may be 0.5 to 5 μm. If thethickness of the ceramic layer 30 is too thin, the insulating effect ofthe ceramic layer 30 may be small. If the thickness of the ceramic layer30 is too thick, the adhesion of the ceramic layer 30 becomes low, andpeeling may occur. Therefore, the thickness of the ceramic layer 30 canbe adjusted to the above-mentioned range. More specifically, thethickness of the ceramic layer 30 may be 0.8 to 3.2 μm.

The ceramic layer 30 may comprise a ceramic powder. The ceramic powdermay be at least one selected from Al₂O₃, SiO₂, TiO₂, ZrO₂, Al₂O₃.TiO₂,Y₂O₃, 9Al₂O₃.2B₂O₃, BN, CrN, BaTiO₃, SiC, and TiC. The particle diameterof the ceramic powder may be 2 to 900 nm. If the particle diameter ofthe ceramic powder is too small, the formation of the ceramic layer maybecome difficult. If the particle diameter of the ceramic powder is toolarge, the surface roughness may become high and surface defects mayoccur. Therefore, the particle diameter of the ceramic powder can beadjusted to the above-mentioned range.

The ceramic powder may be in the form of any one or more selected fromthe group including spherical, plate-like, and needle-shaped.

The ceramic layer 30 may further comprise a metal phosphate. The metalphosphate may include at least one selected from Mg, Ca, Ba, Sr, Zn, Aland Mn. When the ceramic layer further includes the metal phosphate, theinsulating property of the ceramic layer 30 is further improved.

The metal phosphate may be composed of a compound by chemical reactionof metal hydroxide and phosphoric acid (H₃PO₄).

The metal phosphate may be a compound formed by chemical reaction ofmetal hydroxide and phosphoric acid (H₃PO₄). The metal hydroxide may beat least one selected from the group including Ca(OH)₂, Al(OH)₃,Mg(OH)₂, B(OH)₃, Co(OH)₂, and Cr(OH)₃.

Specifically, the metal atom of the metal hydroxide may be composed offorming a single bond, a double bond, or a triple bond by a substitutionreaction with phosphorous of phosphoric acid, and a compound wherein theamount of unreacted free phosphoric acid (H₃PO₄) is 25 wt. % or lower.

The metal phosphate may be a compound formed by a chemical reactionbetween metal hydroxide and phosphoric acid (H₃PO₄), and the weightratio of metal hydroxide to phosphoric acid may be 1:100 to 40:100.

If the amount of the metal hydroxide is too much, the chemical reactionmay not be completed and the problem of generating precipitates mayoccur. If the metal hydroxide is contained too little, the problem ofcorrosion resistance may occur. Thus, the amount may be limited to theabove-mentioned range.

The reason for limiting the components of the grain-oriented electricalsteel sheet substrate 10 will be described in the below.

Si: 2.8 to 6.8% by weight

Silicon (Si) increases the resistivity of the steel to reduce iron loss.When the content of Si is too small, the resistivity of the steelbecomes small and the iron loss characteristic deteriorates. Further, inthe high-temperature annealing, the phase transformation zone exists,thereby causing the problem of destabilizing the secondrecrystallization. If the content of Si is too large, the brittlenessmay be increased and cold-rolling may become difficult. Therefore, thecontent of Si can be controlled within the above-mentioned range. Morespecifically, Si may be contained in an amount of 3.8 to 5.8% by weight.

Al: 0.020 to 0.040% by weight

Aluminum (Al) is a component that acts as an inhibitor by finally beingnitride of AlN, (Al,Si)N, (Al,Si,Mn)N type. When the content of Al istoo small, it is difficult to expect a sufficient effect as aninhibitor. When the content of Al is too large, the Al-based nitrideprecipitates and grows too large. Thus, the effect as an inhibitor maybecome insufficient. Therefore, the content of Al can be controlledwithin the above-mentioned range.

Mn: 0.01 to 0.20% by weight

Manganese (Mn) has the effect of reducing the iron loss by increasingthe resistivity as Si and reacting with the nitrogen introduced by thenitriding treatment together with Si to form precipitates of(Al,Si,Mn)N. It is an important element for suppressing the growth ofthe primary recrystallization grains and causing the secondaryrecrystallization. However, when the content of Mn is too large, itaccelerates the Austenite phase transformation during the hot-rolling sothat the size of the primary recrystallization grains is reduced to makethe secondary recrystallization unstable. In addition, when the contentof Mn is too small, the effect of increasing the Austenite fractionduring the hot-rolling reheating as the Austenite forming element toincrease the amount of precipitates and thus to make the primaryrecrystallization through MnS formation, could be insufficient.Therefore, the content of Mn can be controlled within theabove-mentioned range.

Sb, Sn or a combination thereof: 0.01 to 0.15% by weight

Sb or Sn is a grain boundary segregation element which interferes withthe grain boundary movement. It promotes the generation of Goss grain inthe {110}<001> orientation as a grain growth inhibitor, so that thesecondary recrystallization is well developed. Thus, it is important forcontrolling grain size. If the content of Sb or Sn added alone or incombination is too small, the effect may be deteriorated. If the contentof Sb or Sn added alone or in combination is too large, the grainboundary segregation occurs severely and the brittleness of the steelsheet becomes large. Thus, a plate breakage may occur during therolling.

More specifically, it may contain 0.01 to 0.05% by weight of Sb and 0.01to 0.12% by weight of Sn.

C: 0.01% by weight or less

C is a component which does not greatly contribute to the improvement ofthe magnetic properties of the grain-oriented electrical steel sheet inthe embodiment of the present invention, and thus it is preferable toremove C as much as possible. However, if the C content is above acertain level, the Austenite transformation of the steel is promoted inthe rolling. Thus, the hot-rolled structure is refined during thehot-rolling which facilitates the formation of uniform microstructure.

Therefore, the preferred C content in the slab is 0.03 weight % or more.However, when the C content is excessive, coarse carbide is producedwhich is not easily removed during decarburization. Thus, it ispreferable to contain 0.08 wt. % or less of C. The carbon isdecarburized through the decarburization annealing in the process ofmanufacturing the grain-oriented electrical steel sheet. In the finalrain-oriented electrical steel sheet may contain 0.01 wt. % or less ofC.

N: 0.001 to 0.005% by weight

N is an element that reacts with Al or the like to refine the grains.When these elements are appropriately distributed, it is possible toappropriately refine the structure after the cold-rolling as describedabove, thereby ensuring proper primary recrystallization grain size.However, if the content is excessive, the primary recrystallized grainsare excessively refined. As a result, due to the fine grains, thedriving force causing grain growth during the secondaryrecrystallization increases, so that the even grains in undesiredorientation can grow. Also, if the N content is excessive, it takes along time to remove N in the final annealing process, which is notpreferable. Therefore, the upper limit of the nitrogen content should be0.005 wt. %. Further, the nitrogen content should be 0.001 wt. % or morewhen reheating the slab. Thus, the lower limit of the nitrogen contentis preferably 0.001 wt. %. Nitrogen is partially penetrated through thenitriding annealing process in the production of the grain-orientedelectrical steel sheet, and N is included in the final grain-orientedelectrical steel sheet in an amount of 0.005 to 0.05% by weight.

FIG. 3 schematically shows a flow chart of a method of manufacturing agrain-oriented electrical steel sheet according to an embodiment of thepresent invention.

The flow chart of the method of manufacturing the grain-orientedelectrical steel sheet of FIG. 3 is merely for illustrating the presentinvention, and the present invention is not limited thereto. Therefore,the manufacturing method of the grain-oriented electrical steel sheetcan be variously modified.

As shown in FIG. 3, a method of manufacturing a grain-orientedelectrical steel sheet includes: preparing a steel slab (S10); heatingthe steel slab (S20); hot-rolling the heated steel slab to produce ahot-rolled steel sheet (S30); cold-rolling the hot-rolled sheet toproduce a cold-rolled sheet (S40); decarburization annealing andnitriding annealing the cold-rolled steel sheet (S50); applying anannealing separating agent on the surface of the steel sheet subjectedto decarburization annealing and nitriding annealing (S60); andannealing the steel sheet coated with the annealing separating agent ata high temperature (S70). In addition, the manufacturing method of thegrain-oriented electrical steel sheet may further include other steps.

First, in the step S10, a steel slab is prepared. Since the componentsof the steel slab are described in detail with respect to the componentsof the grain-oriented electrical steel sheet described above, repeateddescription is omitted.

Next, in the step S20, the steel slab is heated. At this time, the slabheating can be performed by the low-temperature slab method at 1,200° C.or less.

Next, in the step S30, the heated steel slab is hot-rolled to produce ahot-rolled steel sheet. After the step S30, the produced hot-rolledsheet can be subject to hot-rolled annealing.

Next, in the step S40, the hot-rolled sheet is cold-rolled to produce acold-rolled sheet. In the step S40, the cold-rolling may be performedonce, or the cold-rolling may be performed twice or more includingintermediate annealing.

Next, in the step S50, the cold-rolled sheet can be subject todecarburization annealing and nitriding annealing. At this time, thestep of decarburization annealing and nitriding annealing thecold-rolled sheet can be performed by decarburization annealing andnitriding annealing at the same time, or nitriding annealing afterdecarburization annealing.

Next, in the step S60, the annealing separating agent is applied on thesurface of the steel sheet subject to decarburization annealing andnitriding annealing. Since the annealing separating agent has beendescribed above in detail, repeated description is omitted.

Next, in the step S70, the steel sheet coated with the annealingseparating agent is annealed at a high temperature. During thehigh-temperature annealing, Ca and Mg in the annealing separating agentreact with Si in the grain-oriented electrical steel sheet substrate 10to form the Monticellite coating 20.

The primary cracking temperature and the secondary cracking temperatureare 1200° C. and 700° C., respectively. In the high-temperatureannealing, it is controlled to raise 15° C./hr in the temperatureelevating zone. In addition, the gas atmosphere may be a mixed gasatmosphere of 25% nitrogen and 75% hydrogen until 1200° C., and afterreaching 1200° C., it may be maintained in a 100% hydrogen atmospherefor 15 hours and then furnace-cooled.

After the step S70, the step of forming the ceramic layer 30 may befurther comprised. Since the ceramic layer 30 has been described in theabove in detail, repeated description is omitted. As a method forforming the ceramic layer 30, a ceramic layer may be formed by sprayinga ceramic powder onto the Monticellite coating. Specifically, plasmaspray coating, high velocity oxy fuel coating, aerosol deposition, andcold spray coating can be applied. More specifically, a plasma spraycoating method that a ceramic layer is formed by supplying a ceramicpowder to a heat source in which a gas containing Ar, H₂, N₂, or He isplasma-generated at an output of 20 to 300 kW, may be used. As a plasmaspray coating method, a gas containing Ar, H₂, N₂, or He may be suppliedin a suspension form of a mixture of a ceramic powder and a solvent to aheat source of plasma with an output of 20 to 300 kW to form a ceramiclayer 30. At this time, the solvent may be water or alcohol.

As a method for forming the ceramic layer 30, a method of forming aceramic layer by applying a composition for forming a ceramic layercontaining ceramic powder and metal phosphate may be used.

After formation of the ceramic layer 30, magnetic domain refining can beperformed as required.

Hereinafter, the present invention will be described in more detail withreference to examples. However, these embodiments are only forillustrating the present invention, and the present invention is notlimited thereto.

Example 1: Properties of Ceramic Powders Example 1

The slab consisting of 3.4 wt. % of silicon (Si), 0.03 wt. % of aluminum(Al), 0.05 wt. % of manganese (Mn), 0.04 wt. % of antimony (Sb), and0.11 wt. % of tin (Sn), 0.06 wt. % of carbon (C), and 40 weight ppm ofnitrogen (N), and the remainder of Fe and other unavoidable impurities,was prepared.

The slab was heated at 1150° C. for 220 minutes and hot-rolled to athickness of 2.3 mm to prepare a hot-rolled sheet.

The hot-rolled sheet was heated to 1120° C., held at 920° C. for 95seconds, quenched in water and pickled, and then cold-rolled to athickness of 0.23 mm to prepare a cold-rolled sheet.

The cold-rolled sheet was put into a furnace maintained at 850° C., andthen the dew point and the oxidizing ability were controlled. Thedecarburization and the primary recrystallization annealing wereperformed simultaneously in a mixed gas atmosphere of hydrogen,nitrogen, and ammonia to prepare a decarburized and annealed steelsheet.

As an annealing separating agent composition, 50% by weight of calciumtitanate (CaTiO₃), 40% by weight of magnesium oxide, 5% by weight oftitanium oxide, and 5% by weight of Sb₂(SO₄)₃ were mixed with distilledwater to prepare slurry form. The slurry was applied to a decarburizedand annealed steel sheet, and then a final annealing was conducted.

During the final annealing, the primary cracking temperature was 700°C., the secondary cracking temperature was 1200° C., and the temperaturewas raised 15° C./hr in the temperature elevating zone. In addition, upto 1200° C., a mixed gas atmosphere of 50 vol % of nitrogen and 50 vol %of hydrogen was used. After reaching 1200° C., it was maintained in 100vol % of hydrogen gas atmosphere for 20 hours and then furnace cooled.

The Monticellite coating prepared through the final annealing wasquantitatively analyzed by X-Ray Diffraction (XRD) and the results areshown in FIG. 4. The SEM EDS analysis results of the Monticellitecoating are shown in FIG. 6. As shown in FIG. 6, in the Monticellitecoating, it was analyzed that 11.27% by weight of Ca, 8.23% by weight ofMg, 8.30% by weight of Si, and 7.45% by weight of O, were contained.

Thereafter, TiO₂ was supplied as a ceramic powder to a heat source inwhich argon (Ar) gas was converted into plasma at an output of 250 kW toform a ceramic layer having a thickness of 0.9 μm on the surface of thefinal annealed plate.

FIG. 5 shows a Scanning Electron Microscope (SEM) photograph of thegrain-oriented electrical steel sheet produced in Example 1. It can beconfirmed that a Monticellite coating containing a calcium component anda ceramic layer were sequentially formed on the grain-orientedelectrical steel sheet substrate.

Examples 2-12

The same procedure as in Example 1 was carried out except that thecalcium compound and the ceramic powder in the annealing separatingagent were replaced with the calcium compound and the ceramic powder setforth in Table 1 below to form a Monticellite coating and a ceramiclayer.

It was confirmed that Monticellite coating was formed in all the calciumcompounds.

Example 13

The same procedure as in Example 1 was carried out except that noceramic layer was formed.

Comparative Example 1

The same procedure as in Example 13 was carried out except that theannealing separating agent composition including 90% by weight ofmagnesium oxide, 5% by weight of titanium oxide, and 5% by weight ofSb₂(SO₄)₃ was used.

Comparative Example 2

The same procedure as in Example 1 was carried out except that theannealing separating agent composition including 90 wt. % of magnesiumoxide, 5 wt. % of titanium oxide, and 5 wt. % of Sb₂(SO₄)₃ was used.

FIG. 7 shows Fourier Transform Infrared Spectroscopy (FT-IR) analysisresults of the Monticellite coating and the Fosterite coating preparedin Example 1 and Comparative Example 2.

The magnetic steel sheets prepared in Examples and Comparative Exampleswere evaluated for magnetic properties and noise characteristics underthe conditions of 1.7 T and 50 Hz, and the results are shown in Table 1below.

The magnetic properties of the electric steel sheet are usuallyexpressed by W17/50 and B8. W17/50 means the power loss when a magneticfield of 50 Hz frequency is magnetized to AC up to 1.7 Tesla. Here,Tesla is a unit of magnetic flux density, which means flux per unitarea. B8 shows the magnetic density value of flux flowing through theelectric steel sheet when a current of 800 A/m is applied to the coilwound around the electric steel sheet.

In addition, the insulation properties were measured using a Franklinmeter according to ASTM A717 International Standard.

Further, the adhesion is represented by the minimum arc diameter withoutpeeling of the coating when the specimen is bent by 180° in contact withthe arc of 10 to 100 mm.

TABLE 1 Magnetic Flux Calcium Ceramic Iron Loss Density InsulationAdhesion Compound Powder (W17/50) (B8) (mA) (mmφ) Example 1 CaTiO₃ TiO₂0.650 1.916 420 20 Example 2 CaO SiO₂ 0.770 1.907 520 35 Example 3Ca(OH)₂ Al₂O₃ 0.634 1.922 340 20 Example 4 Ca₃Co₄O₉ ZrO₂ 0.752 1.904 61525 Example 5 CaSiO₃ Al₂O₃•TiO₂ 0.682 1.932 440 15 Example 6 CaZrO₃ Y₂O₃0.711 1.935 615 15 Example 7 Ca₅(OH)(PO₄)₃ 9Al₂O₃ •2B₂O₃ 0.655 1.945 21020 Example 8 CaCO₃ BN 0.764 1.909 820 15 Example 9 CaH₂ CrN 0.710 1.905790 15 Example 10 CaC₂ BaTiO₃ 0.815 1.911 120 20 Example 11 Ca₃(PO₄)₂SiC 0.789 1.915 350 25 Example 12 CaSO₄ TiC 0.750 1.910 465 25 Example13 CaTiO₃ — 0.920 1.913 750 15 Comparative — — 0.981 1.910 982 30Example 1 Comparative — TiO₂ 0.765 1.915 670 25 Example 2

As shown in Table 1, it can be confirmed that the properties of Examples1 to 13 are superior to those of Comparative Examples 1 and 2.

Experimental Example 2: Evaluation of Magnetic Characteristics, SpaceFactor and Noise Characteristics of a 1000 kVA Transformer Example 14

The slab consisting of 3.3 wt. % of silicon (Si), 0.03 wt. % of aluminum(Al), 0.03 wt. % of antimony (Sb), 0.06 wt. % of tin (Sn), 0.05 wt. % ofcarbon (C), and 30 weight ppm of nitrogen (N), and the remainder of Feand other unavoidable impurities, was prepared.

The slab was heated at 1150° C. for 220 minutes and hot-rolled to athickness of 2.3 mm to prepare a hot-rolled sheet.

The hot-rolled sheet was heated to 1120° C., held at 920° C. for 95seconds, quenched in water and pickled, and then cold-rolled to athickness of 0.23 mm to prepare a cold-rolled sheet.

The cold-rolled sheet was put into a furnace maintained at 850° C., andthen the dew point and the oxidizing ability were controlled. Thedecarburization and the primary recrystallization annealing wereperformed simultaneously in a mixed gas atmosphere of hydrogen,nitrogen, and ammonia to prepare a decarburized and annealed steelsheet.

As an annealing separating agent composition, 50% by weight of calciumtitanate (CaTiO₃), 40% by weight of magnesium oxide, 5% by weight oftitanium oxide, and 5% by weight of Sb₂(SO₄)₃ were mixed with distilledwater to prepare slurry form. The slurry was applied to a decarburizedand annealed steel sheet, and then a final annealing was conducted.

During the final annealing, the primary cracking temperature was 700°C., the secondary cracking temperature was 1200° C., and the temperaturewas raised 15° C./hr in the temperature elevating zone. In addition, upto 1200° C., a mixed gas atmosphere of 50 vol % of nitrogen and 50 vol %of hydrogen was used. After reaching 1200° C., it was maintained in 100vol % of hydrogen gas atmosphere for 20 hours and then furnace cooled.

Thereafter, the composition for forming a ceramic layer mixed with 45%by weight of colloidal silica, 45% by weight of aluminum phosphate, 5%by weight of chromium oxide, and 5% by weight of nickel hydroxide wasstirred. The composition was applied 4.5 g/m² on the final annealedplate. And then, the plate was processed in a drying furnace set at 860°C. for 120 seconds, followed by a laser magnetic domain refiningprocess. A 1000 kVA transformer was manufactured and evaluated at 60 Hzaccording to the design flux density. The results were shown in Table 2below.

Comparative Example 3

The same procedure as in Example 14 was carried out except that theannealing separating agent composition including 90% by weight ofmagnesium oxide, 5% by weight of titanium oxide, and 5% by weight ofSb₂(SO₄)₃ was used.

The space factor was measured using a measuring instrument according tothe JIS C2550 International Standard. A uniform pressure of 1 MPa wasapplied to the surface after laminating a plurality of electric steelplate specimens. And then, the ratio of the real weight according to theelectric steel plate lamination to the theoretical weight was measuredby the precision measurement of the height of the four faces of thespecimen.

The noise is evaluated in the same way as the IEC61672-1 InternationalStandard. However, instead of the sound pressure, the vibration data ofthe electric steel sheet is obtained and evaluated as the noiseconversion value [dBA]. The vibration of the electric steel sheet ismeasured by the non-contact type vibration pattern by using the laserDoppler method when the magnetic field of the frequency 60 Hz ismagnetized to AC of 1.7 Tesla.

TABLE 2 Magnetic Space Iron Loss Flux Density Factor Noise Coating(W17/50) (B8) (%) (dBA) Example 14 Monticellite 0.760 1.915 97.5 51.4Comparative Fosterite 0.842 1.908 96.2 55.5 Example 3

As shown in Table 2, it can be seen that the characteristics of Example14 are far superior to those of Comparative Example 3.

It will be understood by those of ordinary skill in the art that variouschanges in form and details may be made herein without departing fromthe spirit and scope of the present invention as defined by thefollowing claims and their equivalents. It will be understood that theinvention may be practiced. It is therefore to be understood that theabove-described embodiments are illustrative in all aspects and notrestrictive.

DESCRIPTION OF SYMBOLS

-   100: Grain-oriented electrical steel sheet 10: Grain-oriented    electrical steel sheet-   20: Monticellite coating 30: Ceramic layer

What claimed is:
 1. An annealing separating agent composition for agrain-oriented electrical steel sheet comprising, based on the solidscontent, 30 to 70% by weight of a calcium compound, and magnesium oxideor magnesium hydroxide in a balance.
 2. The annealing separating agentcomposition for a grain-oriented electrical steel sheet according toclaim 1, further comprising 1 to 10% by weight of the ceramic powder. 3.The annealing separating agent composition for a grain-orientedelectrical steel sheet according to claim 2, wherein the ceramic powderis at least one selected from the group consisting of Al₂O₃, SiO₂, TiO₂,and ZrO₂.
 4. The annealing separating agent composition for agrain-oriented electrical steel sheet according to claim 1, furthercomprising Sb₂(SO₄)₃, SrSO₄, BaSO₄, or a combination thereof in anamount of 1 to 10% by weight.
 5. The annealing separating agentcomposition for a grain-oriented electrical steel sheet according toclaim 1, wherein the calcium compound is at least one selected from thegroup consisting of calcium oxide (CaO), calcium hydroxide (Ca(OH)₂),calcium cobalt oxide (Ca₃Co₄O₉), calcium silicate (CaSiO₃), calciumtitanate (CaTiO₃), calcium zirconate (CaZrO₃), hydroxyapatite(Ca₅(OH)(PO₄)₃), calcium carbonate (CaCO₃), calcium hydride (CaH₂),calcium carbide (CaC₂), calcium phosphate (Ca₃(PO₄)₂), calcium sulfate(CaSO₄), calcium oxylate (CaC₂O₄), calcium peroxide (CaO₂), and calciumchromate (CaCrO₄).
 6. A grain-oriented electrical steel sheet, having aMonticellite coating formed on one side or both sides of agrain-oriented electrical steel sheet substrate.
 7. The grain-orientedelectrical steel sheet according to claim 6, wherein the Monticellitecoating contains 0.5 to 90% by weight of Ca.
 8. The grain-orientedelectrical steel sheet according to claim 7, wherein the Monticellitecoating further comprises Mg in an amount of 3 to 80% by weight, Si inan amount of 3 to 80% by weight, O in an amount of 3 to 80% by weight,and Fe in a balance.
 9. The grain-oriented electrical steel sheetaccording to claim 6, wherein the Monticellite coating has a thicknessof 0.1 to 10 μm.
 10. The grain-oriented electrical steel sheet accordingto claim 6, wherein a ceramic layer is further formed on theMonticellite coating.
 11. The grain-oriented electrical steel sheetaccording to claim 10, wherein the ceramic layer comprises a ceramicpowder.
 12. The grain-oriented electrical steel sheet according to claim11, wherein the ceramic powder is at least one selected from the groupconsisting of Al₂O₃, SiO₂, TiO₂, ZrO₂, Al₂O₃, TiO₂, Y₂O₃, 9Al₂O₃.2B₂O₃,BN, CrN, BaTiO₃, SiC, and TiC.
 13. The grain-oriented electrical steelsheet according to claim 11, wherein the ceramic layer further comprisesa metal phosphate.
 14. The grain-oriented electrical steel sheetaccording to claim 13, wherein the metal phosphate comprises at leastone selected from Mg, Ca, Ba, Sr, Zn, Al, and Mn.
 15. The grain-orientedelectrical steel sheet according to claim 6, wherein the grain-orientedelectrical steel sheet substrate comprises silicon (Si): 2.8 to 6.8 wt.%, aluminum (Al): 0.020 to 0.040 wt. %, manganese (Mn): 0.01 to 0.20 wt.%, and antimony (Sb), tin (Sn), or a combination thereof in an amount of0.01 to 0.15 wt. %, Fe and other unavoidable impurities in a balance.16. A manufacturing method of a grain-oriented electrical steel sheetcomprising: preparing a steel slab; heating the steel slab; hot-rollingthe heated steel slab to produce a hot-rolled steel sheet; cold-rollingthe hot-rolled sheet to produce a cold-rolled sheet; decarburizingannealing and nitriding annealing the cold rolled sheet; applying anannealing separating agent on the surface of the decarburizing annealedand nitriding annealed steel sheet; and high-temperature annealing thesteel sheet applied with the annealing separating agent, wherein theannealing separating agent comprises, based on a solid content, 30 to70% by weight of a calcium compound, and magnesium oxide or magnesiumhydroxide in a balance.
 17. The manufacturing method of a grain-orientedelectrical steel sheet according to claim 16, further comprising a stepof forming a ceramic layer on the Monticellite coating after thehigh-temperature annealing.
 18. The manufacturing method of agrain-oriented electrical steel sheet according to claim 17, wherein thestep of forming the ceramic layer is a step of forming a ceramic layerby spraying a ceramic powder onto the Monticellite coating.
 19. Themanufacturing method of a grain-oriented electrical steel sheetaccording to claim 17, wherein the step of forming the ceramic layer isa step of forming a ceramic layer by applying a composition for forminga ceramic layer comprising a ceramic powder and a metal phosphate on theMonticellite coating.
 20. The manufacturing method of a grain-orientedelectrical steel sheet according to claim 16, wherein the step ofdecarburization annealing and nitriding annealing the cold-rolled sheetis a step of decarburization annealing and nitriding annealing thecold-rolled sheet simultaneously, or nitriding annealing the cold-rolledsheet after decarburization annealing.